1. Field of the Invention
The present invention relates to an electronic cassette used for radiography and an operation method of an electronic cassette.
2. Description Related to the Prior Art
In medical radiography, e.g., radiography, an electronic cassette is used widely. The electronic cassette has a constitution that an image detection unit (it is also called a flat panel detector (FPD)) which detects an X-ray image of a subject such as a patient is accommodated within a housing of portable type.
For example, the image detection unit is composed of a scintillator which converts X-rays into visible light and emits it and a photo detection substrate which detects the emitted visible light from the scintillator and converts it into an electric signal. The photo detection substrate includes a plurality of pixels arranged two-dimensionally. The pixel is composed of a photoelectric converter which responds to visible light and produces and accumulates electric charge, and a switching element such as a TFT (Thin Film Transistor) which is connected to the photoelectric converter for reading the electric charge accumulated in the photoelectric converter. A bias voltage is applied to the photoelectric converter from a bias power circuit.
As stated in Japanese Patent Laid-Open Publication No. 2014-160046, there is an electronic cassette usable without a cable, by comprising a radio communication unit performing radio communication with an external equipment such as a control unit controlling operations of the electronic cassette and a battery to supply electricity to drive the electronic cassette. In Japanese Patent Laid-Open Publication No. 2014-160046, the battery supplies electricity to sections of the electronic cassette such as a bias power circuit.
As stated in Japanese Patent Laid-Open Publication No. 2014-142355, a battery is detachably attached in a housing. When remaining capacity of the battery has reduced, an operator such as a radiology technician removes the battery from the housing and replaces with another battery which has been charged.
In addition, Japanese Patent Laid-Open Publication No. 2014-160046 states that an offset correction is performed to remove influence of a fixed pattern noise due to operating environment of the electronic cassette from an X-ray image. As is generally known, the offset correction is a process that the image detection unit detects an image (image for offset correction) in the state that X-rays are not irradiated, and the image for offset correction is deducted pixel-by-pixel basis from an X-ray image which the image detection unit detected based on X-rays which transmitted through an object.
When the battery is detached from the housing for exchange, since electricity is not supplied to the bias power circuit and other sections, a bias voltage is not applied to the photoelectric converter. Therefore, it takes a long start-up time from turning on of the main power of the electronic cassette after the exchange of the battery to a state that photographing preparation is completed to perform X-rays photography. The reason is that a detection of an image for offset correction should be performed again when an application of bias voltage is stopped once and then re-started, and it takes a long time for stabilization of the operation of the photoelectric converter which is necessary to detect the image for offset correction precisely.
So far, when a battery is exchanged in the middle of radiography, re-start of the radiography is delayed due to the long start-up time, and a stress to an object and a reduction in photography efficiency are caused. Therefore, it has been demanded shortening of the start-up time when a battery is exchanged.